1. Field of the Invention
The present invention relates to automatic replenishment systems for processors of photosensitive material. In particular, the present invention relates to an improved film width and transmittance scanning system for controlling developer and fix replenishment in a graphic arts film processor.
2. Description of the Prior Art
Graphic arts film processors require replenishment of developer and fixer to compensate for change in the chemical activity resulting from the processing of photosensitive film. Replenishment systems were originally manually operated. The operator would visually inspect the film being processed and would manually operate the replenishment systems as he deemed necessary. The accuracy of these types of manual replenishment systems was obviously based upon the skill of the operator.
In recent years, automatic replenishment systems have found increasing use. These systems typically utilize film transmittance measurements to control the operation of the replenishment system. Examples of automatic replenishment systems are shown in U.S. Pat. No. 4,104,670 to Charnley et al, U.S. Pat. No. 4,057,818 to Gaskell et al, U.S. Pat. No. 4,128,325 to Melander et al, U.S. Pat. No. 4,174,169 to Melander et al, and U.S. Pat. Nos. 4,293,211, 4,295,729, 4,314,753, 4,332,456, 4,346,981, 4,372,665, 4,372,666 and 4,422,152 to Kaufmann. All of these patents are assigned to the same assignee as the present application. Other examples of automatic replenishment systems may be found in U.S. Pat. No. 3,472,143 to Hixon et al, U.S. Pat. No. 3,554,109 to Street et al, U.S. Pat. No. 3,559,555 to Street; U.S. Pat. No. 3,561,344 to Frutiger et al, U.S. Pat. No. 3,696,728 to Hope, U.S. Pat. No. 3,787,689 to Fidelman, U.S. Pat. No. 3,927,417 to Kinoshita et al, U.S. Pat. No. 4,119,952 to Takahashi et al and U.S. Pat. No. 4,134,663 to Laar et al.
In a typical graphic arts automatic replenishment system, a scanner is used to measure transmittance of the film after it has been developed. The scanner includes a light source positioned on one side of the film path, and a light receiver positioned on the opposite side of the film path. The amount of light which passes from the light source to the receiver is modulated by the film passing inbetween. This is a measurement of transmittance (T), which is the ratio of "transmitted" to "initial" illuminance.
There are two basic types of film that are processed by a graphic arts processor: half-tone film and continuous-tone flm. Half-tone film consists of varying sizes of discrete dots, while continuous-tone film is based on continuous variation of the transmissive qualities of the film. Because of the different properties of half-tone and continuous-tone film, the same average transmittance, as measured by a scanner, will require markedly different amounts of developer replenishment.
In half-tone film, if a spot is developed at all, it is completely developed, and if a spot is clear, it is completely clear. For example, for a film with "30% dot", thirty percent of the silver has been developed, and this covers thirty percent of the film. With thirty percent of the surface of the film opaque, thirty percent of the incident light will be blocked and seventy percent of the incident light will be detected by the sensing strip. In terms of the definition of transmittance T, if the light source prior to film arrival ("no film") is normalized to one hundred percent (100%), and seventy percent (70%) of the light is transmitted when film is present, then the ratio of the transmittance to the initial illuminance is seventy percent (i.e. T=70%).
Developer replenishment is based on the amount of silver that was developed and is blocking light. In a half-tone film, developer replenishment is proportional to the percentage dot, that is proportional to one hundred percent minus average percentage transmittance. In the example given above, in which transmittance is seventy percent, thirty percent of the maximum developer replenishment volume recommended for a totally exposed film must be used.
As a first approximation for continuous-tone film, optical density D is proportional to the amount of silver present in the image, and density is therefore the appropriate number to use to measure the photographic effect. This is an approximation because the D log E curve flattens out on both ends and is not a pure straight line for the full range of the long scale. Density is a logarithm (to the base 10) of the opacity (where opacity is the reciprocal of the transmittance). In other words: EQU D=log.sub.10 (1/T)
For example, using a 40% transmittance, density equals 0.22. For any given type of continous-tone film, there is a maximum obtainable density value associated with it, where the D log E curve flattens out. This is considered a fully exposed film, and results in a density of approximately 2.00 to 4.00 for most films, although it can be higher or lower depending on film type.
If the film type specifies a maximum density and a corresponding replenishment rate for fully exposed film, a proportional replenishment volume can then be derived. For example, if the maximum density is D=2.00 and replenishment volume for fully exposed film equals 1 cc/square inch, a sensed transmittance of 40% (resulting in a computed density of 0.22) results in a replenishment rate of 0.11 cc/square inch.
By referencing a table that converts densitometer density readings of a half-tone film to the actual percentage dot of that film, it is possible to compare continuous tone and half tone films and their required replenishment rates. With the understanding that continuous tone replenishment is proportional to density and half-tone replenishment is proportional percentage dot, the following example will compare different density levels between the two types of film.
If the same replenishment rate X is given for a 50% dot for half-tone and 0.301 density for continous tone, the following table relates the replenishment volume and other points of percentage dot versus the 50% level.
______________________________________ HALF TONE CONTINUOUS REPLENISH- REPLENISH- TONE MENT MENT "DENSITY" VOLUME % DOT VOLUME ______________________________________ 0.046 (0.153)*(X) 10 (0.20)*(X) 0.097 (0.322)*(X) 20 (0.40)*(X) 0.155 (0.515)*(X) 30 (0.60)*(X) 0.222 (0.738)*(X) 40 (0.80)*(X) 0.301 (1.000)*(X) 50 (1.00)*(X) 0.398 (1.322)*(X) 60 (1.20)*(X) 0.523 (1.738)*(X) 70 (1.40)*(X) 0.699 (2.322)*(X) 80 (1.60)*(X) 1.000 (3.322)*(X) 90 (1.80)*(X) ______________________________________
The worst case listed above is at the 90% dot level, where continous-tone film requires almost twice the replenishment as half-tone film even though the transmittance is the same.
It should be noted that by selecting the 50% dot level as the point of reference, the magnitude of the deviation between the two types of film has been minimized. If a point is picked at the low end of the table as the reference point, the deviation is even greater. Assume, for example, that a 5% dot half-tone film and a continous-tone file of density 0.022 both require X volume of replenishment. At 95% dot, the half-tone film will require (95/5)*(X)=19X replenishment. The continuous-tone time at the same densitometer reading would require (1.301/0.022)*(X)=59X replenishment. In other words, the continuous-tone film would require more than 3 times as much replenishment as half-tone film for the same effective change in the amount of light transmitted through the film.
In other words, continuous-tone film and half-tone film cannot be accurately replenished on the same basis with the same ratio. Half-tone replenishment is proportional to percentage dot, while continuous-tone replenishment is proportional to density.
Conventional scanners used in graphic arts film processors, while termed "density" scanners, are in fact transmittance sensing devices. The film passes between a light source and a light receiver. The scanner output is the integral of light transmitted minus light blocked over the width of the scanner. The scanner output signal is then integrated over the length of the film to produce an integrated signal which represents 100%-%T=% dot.
By design, these types of scanners are incapable of accurately replenishing continuous-tone film. The output of the scanner is an integral or summation of the transmittance complement (100%-%T) over the width of the scanner. At this point, the scanner output signal has already been integrated on the basis of percent dot over the entire width of the scanner. Accurate continuous-tone film replenishment, however, should be based on the integral of the densities of various segments of the film, i.e. on a point by point basis.